Electronic devices such as personal computers usually include a plurality of integrated circuit (IC) or semiconductor chips that communicate with each other, for example, via a common bus. Each IC chip has an output circuit (also called an “output buffer”) to drive signals from the IC chip onto the bus, or alternately, directly onto one or more other IC chips. The speed at which an output circuit switches a signal from logic low to logic high, for example, is known as the slew rate of the output circuit, which is typically measured in volts per unit of time. In order to ensure circuit speed compatibility between IC chips and associated buses, the output circuits used in IC chips are typically selected to have a slew rate within a specified range. If an output circuit does not meet the slew rate specification, its host IC chip may not operate at the specified frequency, and may become incompatible with other chips or devices. The extent of symmetry of rising and falling slew rates may also affect. Further, if the slew rate is too high, undesirable noise may be introduced into the output signal. As a result, it is important for the output drivers to maintain the specified rising and falling slew rates.
The slew rates of an output circuit may vary due to variations in the manufacturing process, variations in operating voltage, variations in operating temperature, and variations in the external load capacitance at the output terminal. As the physical dimensions of IC chips become smaller, it becomes more difficult to control the operating characteristics such as slew rates of the chip's transistors. Process variations in the fabrication of semiconductor chips may cause transistors of the same design to behave differently. For example, the amount of current provided by a transistor, which affects its slew rate, is dependent upon a number of factors including transistor size, gate-to-source voltage, and manufacturing-related parameters. Although transistor size and gate-to-source voltage may be well controlled, manufacturing process characteristics typically vary between transistors because of imperfections in available doping technologies and in other manufacturing technologies. As a result, output circuits of the same design and having the same specified operating characteristics may undesirably operate at different speeds and may have slew rates different from those specified.
Further, transistor operating characteristics vary with changes in temperature. Transistors can operate more slowly as the IC chip heats up and, conversely, can operate more quickly as the IC chip cools down. As a result, the slew rate of conventional output circuits undesirably varies with temperature. Changes in the operating temperature of an output driver may cause the output driver's slew rate to drift from its specified slew rate.
Thus, there is a need for an improved output circuit that maintains a specified and preferably symmetric slew rate despite process, voltage, and temperature variations.